1. Field of the Invention
The present invention relates to a technology applicable to an apparatus and a method of generating a multicolor image from image data, and more specifically to a technology of improving the quality of an image to be generated.
2. Description of the Related Art
An image quality improvement technology referred to as a smoothing technology for automatically determining a jaggy image (jags formed as a group of printed dots) frequently generated in printing a character, a lineal drawing, etc. from printing bit map data developed on an image memory; converting the portion on which the jags are generated into the resolution higher than that of the original data; and representing and printing the jags as a visually smooth line, thereby apparently reducing the jags, is applied to a number of monochromatic printing devices such as a monochromatic laser beam printer which is a kind of image generation device used as an output device of a computation system.
FIG. 1 shows the configuration of the laser beam printer to which the above described image quality technology is applied.
The printing data input from an upper computation system, etc. is developed to image data by an image development unit 1001, and stored in image memory 1002.
The image data stored in the image memory 1002 is read to an image quality improvement/laser modulation signal generation circuit 1003. The image quality improvement/laser modulation signal generation circuit 1003 improves the image quality of the image data through the above described smoothing technology, and generates a signal for modulation of a laser beam generated by a laser 1011 in an optical unit 1010 of an image generation unit 1004. The internal configuration of the image quality improvement/laser modulation signal generation circuit 1003 is described later in detail.
A laser modulation signal generated by the image quality improvement/laser modulation signal generation circuit 1003 is input to the image generation unit 1004, and a printing process is performed in a printing medium such as paper, film, etc. according to the laser modulation signal.
The control circuit 1005 controls the image development unit 1001 and the image quality improvement/laser modulation signal generation circuit 1003 to operate these circuits in synchronization with a laser scanning timing signal indicating the scanning timing of a laser beam for one line of image data.
Described below is the internal configuration of the image generation unit 1004.
The optical unit 1010 includes the laser 1011, a polygon mirror 1012, a mirror motor 1013, and a beam detector 1014.
The laser 1011 outputs a laser beam modulated by the laser modulation signal generated by the image quality improvement/laser modulation signal generation circuit 1003. The laser beam is reflected by the polygon mirror 1012, and is lead to a mirror 1021. The polygon mirror 1012 is designed to rotate in a single direction by the mirror motor 1013 such that a laser beam reaching the mirror 1021 repeats linear scanning in a single direction. The laser beam reflected by the polygon mirror 1012 is input to the beam detector 1014 on each scanning cycle, and is detected thereby. The beam detector 1014 outputs a laser scanning timing signal depending on the input detection intervals of a laser beam, and transmits the signal to a control circuit 1005.
The laser beam reflected by the mirror 1021 is emitted to a light-sensitive drum 1022. The laser beam repeats scanning in a single direction vertical to the rotation direction of the light-sensitive drum 1022 at a constant speed, and forms an electrostatic latent image on the light-sensitive drum 1022. Then, a development device 1012 applies toner onto the light-sensitive drum 1022 on which the electrostatic latent image.
Then, the toner applied onto the light-sensitive drum 1022 is transferred to the printing medium which passes through a medium path between the light-sensitive drum 1022 and a transfer roller 1024, and is then fixed to the printing medium by heat and pressure, thereby performing a printing operation.
The improvement of an image performed by the laser beam printer shown in FIG. 1 is described below by referring to FIG. 2.
The image quality improvement/laser modulation signal generation circuit 1003 first discriminates the generation of jags from the arrangement of the dots represented by the image data shown by (A) in FIG. 2 and stored in the image memory 1002. Then, the output timing of a laser modulation signal from the line N to the line N+1 where the generation of jags is detected is adjusted to be the timing as shown by the lines N and N+1 shown by (B) in FIG. 2. A laser beam is modulated according to the laser modulation signal to allow the light-sensitive drum 1022 to form an electrostatic latent image, and to perform a printing operation at the resolution higher than that of the original image data, thereby obtaining a printout with reduced jags shown by (C) in FIG. 2.
FIG. 3 shows the detailed configuration of the image quality improvement/laser modulation signal generation circuit 1003. In FIG. 3, the image quality improvement/laser modulation signal generation circuit 1003 includes an image memory read circuit 1031, a line buffer 1032, an evaluation window extraction circuit 1033, and an amendment signal generation circuit 1034.
The image memory read circuit 1031 reads the printing bit map data developed on the image memory 1002, and transfers it to the line buffer 1032. The line buffer 1032 includes a shift register, and stores the printing bit map data transmitted from the image memory read circuit 1031.
The evaluation window extraction circuit 1033 extracts data in a rectangular area (referred to as a evaluation window) containing a pixel (referred to as a target dot) in the center which is regarded as a target in the data stored in the line buffer 1032, and outputs an extraction pattern arrangement signal indicating the dot arrangement in the evaluation window. The extraction pattern arrangement signal is input to the amendment signal generation circuit 1034.
The amendment signal generation circuit 1034 is used to generate an image of a target dot based on the dot arrangement in the evaluation window, and includes a lookup table containing laser modulation signal pattern data obtained according to an extraction pattern arrangement signal. FIG. 4 shows an example of the correspondence between the dot arrangement pattern in the rectangular area extracted when, in the dot arrangement pattern of the image data shown by (A) in FIG. 2, the dot at the line=N and the dot=M, and the adjacent dots are target dots, and when the size of the evaluation widow is 5×5 dots, and the arrangement pattern of the dots printed on the print medium corresponding to the target dots. When a signal indicating the extraction arrangement pattern shown on the left of each arrow shown by (A) through (I) in FIG. 4 is input, the amendment signal generation circuit 1034 outputs a laser modulation signal according to which the image generation unit 1004 generates an image of the dot pattern shown on the right of each arrow shown by (A) through (I) in FIG. 4. The output laser modulation signal is provided for the optical unit 1010 of the image generation unit 1004.
In the example shown in FIG. 4, the pattern of the target dot and the surrounding dots in the extraction pattern is different from the printing pattern of the target dot shown by (D), (E), (G), and (H) in FIG. 4. The printing dots are designed according to the following regulations.
(D) When the dots of image data (hereinafter referred to as image dots) are not contained in a target dot, and the image dots exist diagonally above to the right of, on the right of, and below the target dot, printing dots occupy one third on the right of the target dot.
(E) When the image dots are contained in the target dot, and the image dots exist above, the diagonally below to the left of the target data, printing dots occupy the center of, and one third on the left of the target dot, that is, occupy two thirds on the left of the target dot.
(G) When the image dots are contained in the target dot, and the image dots exist diagonally above to the right of, and below the target dot, printing dots occupy the center of, and one third on the right of the target dot, that is, occupy two thirds on the right of the target dot.
(H) When the image dots are not contained in the target dot, and the image dots exist above, to the left of, and diagonally below to the left of the target dot, printing dots occupy one third on the left of the target dot.
The rules used in the smoothing technology are not limited to the rules shown in FIG. 4, but the amendment signal generation circuit 1034 also stores a number of correction patterns other than those listed above. These rules are set from experience through trial and error, and not a few rules are different from the above described rules.
In the image quality improvement/laser modulation signal generation circuit 1003 shown in FIG. 3, the target slot in the data stored in the line buffer 1032 is sequentially moved in synchronization with the dot printing timing in the image generation unit 1004. The laser modulation signal is generated and output for the target dot at the actual timing of printing dots corresponding to the target dot. Thus, the laser modulation signal shown by (B) in FIG. 2 is obtained, thereby successfully improving the image quality as shown by (C) in FIG. 2.
A control clock signal generation circuit 1051 shown in FIG. 3 is contained in the control circuit 1005 shown in FIG. 1, and generates a control clock for operating each unit at the above described operation timing in synchronization with the laser scanning timing signal obtained by the optical unit 1010.
Description below is the color printer which is a multicolor image generation apparatus for generating a multicolor image from image data.
A color printer can be a laser beam system, an ink jet system, a heat transfer system, etc. In any of these systems, images printed in the three primary colors, that is, Y (yellow), M (magenta), and C (cyan), are overlapped on a printing medium. In addition to the three primary colors, R (red), G (green) and B (blue) obtained by overlapping any two colors of Y, M, and C, and K obtained by overlapping the three primary colors can represent seven colors (in this example, no gray scale is applied to each color for simple explanation). The colors, Y, M, and C are referred to as the primary colors, and the colors R, G, B, and K represented by overlapping these primary colors are referred to as secondary colors.
FIG. 5 shows an example of the configuration of the color laser beam printer.
In the color laser beam printer shown in FIG. 5, the printing data input from an upper computer system, etc. is analyzed by an image development unit 1101 into bit map data of three primary colors Y, M, and C, and is then developed on an image memory 1102. An image generation unit 1104 performs the image generating process for the three colors Y, M, and C sequentially, and transfers the result to a single printing medium. At this time, a laser modulation signal generation circuit 1103 generates a laser modulation signal using the data read from the respective planes of Y, M, and C of the above described color-analyzed bit map data. The image generation unit 1104 generates an image for each of the colors Y, M, and C. Unlike the laser modulation signal generation circuit 1003 shown in FIG. 1, the laser modulation signal generation circuit 1103 shown in FIG. 5 does not improve the quality of an image using the smoothing technology.
An optical unit 1110 in the image generation unit 1104 includes a laser 1111, a polygon mirror 1112, a mirror motor 1113, and a beam detector 1114. These components are similar to the components of the optical unit 1010 shown in FIG. 1.
The laser beam reflected by a mirror 1121 in the image generation unit 1104 is emitted onto a light-sensitive drum 1122. The laser beam repeats the scanning operation on the light-sensitive drum 1122, and allows the light-sensitive drum 1122 to form the electrostatic latent image of one of the color planes of Y, M, and C. When a development unit 1123 applies the toner of the corresponding color to the electrostatic latent image generated by the light-sensitive drum 1122. Then, the toner applied to the light-sensitive drum 1122 is transferred and fixed onto a single printing medium passing through the path and a transfer roller 1124. The color printing process can be performed by repeatedly performing the above described operation on each of the planes of Y, M, and C.
The color laser beam printer shown on FIG. 5 performs the process of transferring and fixing the toner of the colors of Y, M, and C applied to the light-sensitive drum 1122 on a single printing medium. Another color laser beam printer temporarily transfers the toner of each color applied to the light-sensitive drum 1122 to an intermediate transfer medium, and then simultaneously transfers and fix the toner of respective colors from the intermediate transfer medium to a single printing medium, thereby avoiding the color shift in color printing.
In the color laser beam printer shown in FIG. 5, K (black) is represented by overlapping the three colors Y, M, and C. However, it is difficult to obtain the toner of the ideal three primary colors Y, M, and C. Therefore, the three overlapped colors Y, M, and C do not represent black in many cases. Therefore, a number of color laser beam printer performs the color printing process by overlapping the toner of K (black) in addition to the toner of the three colors Y, M, and C on the printing medium (or an intermediate transfer medium).
FIG. 6 shows an example of the configuration of the color laser beam printer using the toner of the four colors Y, M, C, and K. In FIG. 6, the components also shown in FIG. 5 are assigned the same unit numbers.
With the configuration shown in FIG. 6, the printing data input from an upper computer system, etc. is developed by the image development unit 1101 into the bit map data of black and the bit map data analyzed into the above described three primary colors from an area other than black, and is then developed on an image memory 1102′.
An image generation unit 1104′ includes the development unit 1123 for developing each of the four colors Y, M, C, and K. The image generating process is performed on each of the color planes, and the result is transferred to a single printing medium (or an intermediate transfer medium). In the process of generating images of the colors Y, M, C, and K, the data read from each of the planes of colors Y, M, C, and K of the above described color-analyzed bit map data is used, thereby successfully realizing the multicolor printing.
The problems with the cases in which the quality of an image is improved by the smoothing technology similar to the technology of a monochrome printer are described below by referring to FIGS. 7 and 8.
FIGS. 7 and 8 show the process of developing the color printing data indicating the similar bit arrangement as shown in FIG. 2 as input from an upper computer system, etc. into the bit map data analyzed into the three primary colors Y, M, and C by the image development unit 1101 shown in FIG. 5, and performing the smoothing process on the data of each plane analyzed into the three primary colors according to the similar rules shown in FIG. 4. FIG. 7 shows an example of the case in which the printing data input from an upper computer system, etc. is represented by a pattern of one color of B (blue). FIG. 8 shows an example of the case in which the printing data input from an upper computer system, etc. is represented by a pattern of two colors of B (blue) and R (red).
In the case shown in FIG. 7, B (blue) in the color printing data shown by (A) is color-analyzed into M (magenta) and C (cyan), and is developed on the image memory 1102 as the bit map data for each plane represented by (B). When the smoothing process is individual performed on the data of each plane according to the rule similar to the rule of the monochrome printing shown in FIG. 2, the smoothing result with the dot arrangement shown by (C) in FIG. 7 can be obtained for each plane, and the final printing result as shown by (D) in FIG. 7 can be obtained. Thus, When the printing data input from an upper computer system, etc. is represented by a monochrome pattern, the smoothing technology is applied to each plane obtained by analyzing data into the three primary colors with an excellent printing result with reduced jags according to the same rules as in the monochrome printing process.
On the other hand, the color printing data shown by (A) in FIG. 8, B (blue) is analyzed into M (magenta) and C (cyan), and R (red) is analyzed into Y (yellow) and M (magenta). They are then developed on the image memory 1102 as the bit map data for each plane as shown by (B). When the smoothing technology is applied to the data for each plane according to the rules similar to the rules of the monochrome printing process shown in FIG. 2, the smoothing result with the dot arrangement as shown by (C) in FIG. 8 can be obtained. According to the result, the printing process can be performed with the three primary colors, that is, Y (yellow), M (magenta), and C (cyan), not appearing in the original color printing data as shown by (D) output actually.
Thus, when different colors represented by the same primary color are adjacent to each other, and when a printed image indicates the color not appearing in the original image data but obtained as a result of performing the smoothing process based on the same rules as the monochrome printing process on each plane of Y, M, and C, the edge of the line at the point at which the color not appearing in the original image data actually appears is recognized as a vague image. When the printed image is compared with the printed image obtained without performing the smoothing process, it is often determined that the printed image without the smoothing technology is better. Therefore, it is not determined that the quality of the image can be improved in the above described method.